Since the discovery of high-temperature superconducting (HTS) materials (superconducting above the liquid nitrogen temperature of 77 K) there have been efforts to research and develop various engineering applications using such HTS materials. In thin film superconductor devices and wires, the most progress has been made with fabrication of devices utilizing an oxide superconductor including yttrium, barium, copper and oxygen in the well-known basic composition of YBa2Cu3O7-x (hereinafter referred to as Y123 or YBCO). Much progress has also been made with rare earth elements “RE” substituted for Y. Biaxially textured superconducting metal oxides, such as Y123, have achieved high critical current densities in coated conductor architecture. These wires are often referred to as second generation HTS wires. Thus, Y123 is the preferred material for many applications, including cables, motors, generators, synchronous condensers, transformers, current limiters, and magnet systems for military, high energy physics, materials processing, transportation and medical uses.
Certain challenges in this field include the need for cost effective methods for producing chemically compatible biaxially textured buffer layers, as well as the need to deposit sufficient thickness of the high critical current density (Jc) superconducting layer. Regarding the first objective, it appears that deformation textured substrates with epitaxial buffer layers can be made cost effective. Regarding the need to deposit thick layers of superconductor precursor compositions, a number of techniques have been evaluated. Chemical vapor deposition (CVD) is not considered a competitive method at this time, due to the very high cost of precursor materials. Most physical vapor deposition (PVD) methods, (for example, pulsed laser ablation, reactive sputtering and electron beam evaporation) are limited by deposition rate, compositional control, and high capital costs. A possible economical PVD method would be thermal or electron beam evaporation of the rare earth elements, copper and barium fluoride, known as the “barium fluoride” process. This process appears to be more rapid than direct PVD methods, but capital costs and control system costs are still likely to be too high. Additionally, the deposited precursor composition must subsequently be reacted in a separate furnace system to form the HTS film.
Solution deposition methods have been evaluated, and these appear to offer much lower costs, since vacuum systems are eliminated. Thus, capital costs are not as high, and deposition rates not as low, as other methods using vacuum systems. Trifluoroacetate (TFA) solution processes offer low costs for precursor compositions, high deposition rate, and non-vacuum processing advantages. Such processes are described, for example, in U.S. Pat. No. 5,231,074 to Cima et al., and PCT Publication No. WO 98/58415, published Dec. 23, 1998 and require dissolution of the constituents of the precursor composition to form a solution phase. Both U.S. Pat. No. 5,231,074 and PCT Publication No. WO 98/58415 are hereby incorporated by reference in their entirety.